Heat exchangers were developed many decades ago and they continue to be extremely useful in many applications requiring heat transfer. While many improvements to the basic design have been made, there still exist tradeoffs and design problems associated with the inclusion of heat exchangers within commercial processes.
One of the problems associated with the use of heat exchangers is the tendency toward fouling. Fouling refers to the formation of various deposits and coatings on the surfaces of heat exchangers as a result of process fluid flow and heat transfer. There are various types of fouling including corrosion, mineral deposits, polymerization, crystallization, coking, sedimentation and biological. In the case of corrosion, the surfaces of the heat exchanger can become corroded as a result of the interaction between the process fluids and the materials used in the construction of the heat exchanger. The situation is made even worse due to the fact that various fouling types can interact with each other to cause even more fouling. Fouling can and does result in additional resistance with respect to the heat transfer and thus decreased
heat transfer performance. Fouling may also cause an increased pressure drop in connection with the fluid flowing on the inside of the exchanger.
One type of heat exchanger which is commonly used in commercial equipment is the shell-and-tube exchanger in which one fluid flows on the inside of the tubes, while the other fluid is forced through the shell and over the outside of the tubes. Typically, baffles are placed to support the tubes and to force the fluid across the tube bundle in a desirable manner.
Fouling can be decreased by the use of higher fluid velocities. In fact, one study has shown that a reduction in fouling in excess of 50% can result from a doubling of fluid velocity. While the use of higher fluid velocities can substantially decrease or even eliminate the fouling problem, higher fluid velocities are unfortunately, generally unattainable on the shell side of conventional shell-and-tube heat exchangers because of excessive pressure drops which are created within the system by baffles. Another problem that often arises in connection with the use of heat exchangers is tube vibration damage. Tube vibration is most intense and damage is most likely to occur in cross flow implementations where fluid flow is perpendicular to the tubes, although tube vibration damage can also occur in non-crossflow (i.e. axial) implementations with high fluid velocities.
Many heat exchangers in use today contain baffles. Baffles are interposed in the fluid path in order to provide support for the tubes and to ensure that the fluid on the outside the tubes flows in the desired direction with respect to the tubes. Unfortunately, however, baffles may increase fouling because of the dead zones they create on the shell side of the exchanger where flow is minimal or even non-existent. A further problem encountered in heat exchangers fitted with baffles is that cross flow may result in potential damage to the tubes as a result of flow-induced vibration. In the case of such damage, processes must often be interrupted or shut down in order to repair the device.
Different types of baffles are conventionally used. One type, segmental baffles, is unacceptable for the low-fouling heat exchangers described in our co-pending application because they produce many zones either with low velocity flow or even no flow at all increasing the probability of fouling. Other types (multiple styles including rods, strips, twisted-tubes) may create longitudinal flow in the central area of the exchanger but these technologies lack the inherent strength and flexibility of configuration of the coiled tube supports shown in application Ser. No. 10/209,126 to allow high velocities on the shellside of the exchanger. The present invention is a development of the coiled tube support system which is directly applicable to the triangular tube configuration and which, moreover, allows the design of more compact and less expensive exchangers.
The tube support system with coiled tube supports, described in application Ser. No. 10/209,126, is mostly suitable for the inline tube arrangement although, as described in the application, it may also be used with the triangular tube configuration. In application Ser. No. 10/209,126, the support structure uses spacer coils, which surround each tube in the bundle. For example, with the triangular tube configuration, the coils surround all the tubes in the bundle with coils on adjacent tubes being wrapped in opposite directions (clockwise and counterclockwise) so that they overlap in the inter-tube region and can be welded together to form an integrated, unitary structure.
The shell-and-tube heat exchanger of the present invention employs helically coiled wires to form a spacing and support structure for the tubes arranged in the triangular configuration within the heat exchanger shell. The wire of the coil, which is wound around alternate tubes in the bundle, has a radial thickness (diameter for a circular wire) substantially equal to the space between the heat exchanger tubes. The exchanger, in addition to the coil-encased tubes preferably uses sealing devices of particular configurations to achieve the desired flow patterns. With exchangers of this construction, the potential for dead zones is reduced and the high velocity axial flow that results substantially eliminates fouling problems, and significantly reduces flow-induced tube vibration that can lead to tube damage.
This invention provides easier fabrication as well as a robust design that is needed to operate the shellside at high velocities. This design must use the triangular tube layout. This tube layout is most suitable as it provides the maximum tubecount within a given shell diameter. This exchanger can be provided with a larger number of tie rods than necessary to achieve mechanical integrity of the bundle, which also provides flexibility in achieving the desired shellside velocity by minimizing flow bypassing.